Family history studies showed a familial aggregation of Alzheimer~s disease among first-degree relatives of Alzheimer~s disease probands compared with controls. A case report shows that dementia in Down syndrome may occur without mental retardation. DNA repair studies show defective nucleotide excision repair of free-radical induced bulky lesion in Down syndrome and sporadic and familial Alzheimer~s disease. Apolipoprotein E4 allele in Down syndrome increases the age-related incidence of dementia and shortens life expectancy.